Preheat oven to 350 degrees F. Line a rimmed baking sheet (s) with parchment paper.Add 1/2 cup of granulated sugar to a small bowl. In a medium bowl combine the flour, salt, baking powder and cornstarch. Whisk to combine.
Using an electric mixer (hand held or a bowl of a stand mixer fitted with a paddle attachment) cream butter on high speed. Add the sugar to the creamed butter and beat until light and airy (3-5 minutes).
Beat in the egg and vanilla extract on low speed just until combined.
Slowly add the dry ingredients and beat on medium speed just until combined into a dough. Be sure to scrape the sides of the bowl and bottom of the bowl to combine all ingredients.
Use a 1-tablespoon cookie scoop to scoop out balls of dough. Gently shape dough into a ball between palms of your hands to ensure there are no cracks.
Place the dough ball in a granulated sugar; roll to coat. NOTE: this keeps the dough from being sticky when making the indentation.
Place cookie dough balls on the prepared baking sheet with 2 inches of space in between.
Using the back of a 1 teaspoon measuring spoon press an indentation into the cookie dough. This will slightly flatten the cookie too. Alternatively you can use your thumb or the end of a wooden spoon.
Place the jam in a microwave safe bowl and microwave for 10-12 seconds. Stir vigorously to smooth out the jam.Fill the indentation in the center of each cookie ball (about 1/2 teaspoon of jam per cookie). Bake cookies for 11-13 minutes. The cookies should be colorless on top and only lightly golden brown on the bottom edges of the cookie when baked. Remove and let them sit on the baking sheet for 10 minutes.
Gently transfer cookies to a wire rack until full cooled.
Option: for a wintery look you can dust the final cookies with powdered sugar. Hold a tablespoon upside down over the jam filling and sprinkle the sugar.